The default head bobbing in S.T.A.L.K.E.R. gave me some headaches, and in most modifications I've played so far it was never changed, so I changed it myself, it's much better now, I can play for hours non-stop without the slightest of the problems. For those who would like to try it you need to find the effectors.ltx file under [...]\THQ\S.T.A.L.K.E.R. - Shadow of Chernobyl\gamedata\config\misc\
Inside that file the very first lines look like this:
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Actor Effector Section
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
[bobbing_effector]
run_amplitude = 0.0025
walk_amplitude = 0.004
limp_amplitude = 0.010
The numbers you see are the ones I've been using (they are modified, so the default values are different). Those values are the only ones that need to be changed to reduce or augment the head bobbing effect. Once the tweaking is done simply save the changes and play the game, there's no need to start a new game for the effect to be applied.
NOTE: If you do not have any modifications installed that modifies the effectors.ltx file then of course that file won't be present at all, and in fact the actual gamedata folder itself won't be present in your main installation directory either. If you do not intend to use any modifications but still want to give a try to the changed head bobbing effect then all you have to do is to create the gamedata folder, and after that of course add inside it the config folder, and then the misc folder... and finally add the effectors.ltx inside and voila it's done.
If you don't feel like finding a modified effectors file around the web or if you don't feel like extracting the default data files of the game (the compressed ones) to find the default effectors then let me know it, I will send you my own effectors via e-mail, just send me a PM about it with your public e-mail address.